Is It Down Right Now? - Network & Admin
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.
RedGate SQL Monitor - SQL Monitor helps you and your team find issues – before they become problems
IsThisSiteDown.org - Free tool to check whether a site is down just for you, or for everyone.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services